PREAMBLE
This thread is intended to provide a framework for planning and executing performance modifications to Porsche 924s. Since the forum doesn’t support “wiki” functionality, I’ve broken the content into separate articles which I’ve submitted under separate posts in the same thread. Since the thread is focused on performance enhancements, I have intentionally left out content that is not directly related to improving or optimizing performance (e.g. fire suppression, safety equipment, audio systems, cosmetic treatments, etc.). And considering I do not race competitively, I have made no effort to account for the various rules promulgated by different sanctioning bodies…you’re on your own in that department!

Many of the discussions on this forum involve people that are new to performance mods in general, and many who have experience with muscle cars or so-called “ricers” and mistakenly try to apply principles that work for those kinds of cars to the 924. The 924 is a unique beast. It comes with a very interesting set of pre-conditions and constraints that have a significant impact on the dollar-to-power ratio one can hope to achieve. There are very few bolt-on performance modifications that equal the results one might experience with other cars.

The 924 demands that you embrace it as an entire platform. At some later date, I may go back and cross-reference all of the pre-requisite reading, most of which can be found by reading the technical FAQ and using the search feature. For now, I will focus on providing a beginning point for addressing the “whole platform” and a systematic approach to evaluating, planning and documenting modifications.

Without such a plan, my experience was that I spent more than I needed to on some components, made early design choices that I later regretted because I made them without considering their relationship to other factors, and worst of all, I cannibalized my own project budget on parts or modifications that weren’t directly related to the plan or were purchased at the wrong time. Having said that, this approach has evolved over time, and will probably continue to evolve. If you’re interested in seeing how I’ve applied some of these principles to my Ultra Wide Body (UWB) project, you might be interested in reading this thread:

WHOLE PLATFORMState Your Objective
Before thinking about your first dollar spent, you should document your goals, objectives and desired outcome. Do you want a daily driver that will look stock but have just that little extra oomph? Do you want a sleeper car that looks stock from the outside but dazzles through the twisties? Do you want a highly competitive DE or AutoX car that can be driven to and from the track? Do you want to race in an SCCA class? Do you want a show car? Do you want a money-no-object street legal track car? Settling on and visualizing the end result based on how you want to use the car is the first step.

You should also think about timing. If you’re modifying your daily driver, having your car out of commission for weeks or months is simply not an option, and will affect not only what you can do, but how much you can achieve at a time. On the other hand, for monster all-inclusive projects, it’s important to at least have target dates to shoot for. When working with vendors, one of the most frustrating aspects is not receiving your stuff on the date promised. Having a target date to communicate to them will at least give you some leverage for pressing them to deliver on a timely basis. It will also help you stay focused on milestones and incremental progresses, rather than losing interest when delays or lags occur.

Establish an Engineering Aesthetic
Next, you need to think about the philosophical approach, the engineering aesthetics if you will, or the design constraints that you want to work within. Are you willing to perform major cosmetic surgery, or do you want to keep the car as close to stock as possible? Do you care about period-correctness, or do you want to modernize as much as possible? Do you care about staying within the Porsche family of components or is hybridization OK?

Obviously, these are personal preference. There is no right way of doing things. But there may be conflicting constraints that will cause you to have to compromise your aesthetics along the way. For example, you simply cannot install a front mount intercooler in a 931 without performing major cosmetic surgery. You need to reach some conclusion about how to balance your performance goals with the aesthetic constraints you either chose or are forced to accept.

Most importantly, write this stuff down so you can refer back to it and remind yourself what you originally set out to accomplish!

Budget in Stages
Next, you need to at least have an idea of what your budgetary constraints are. Invariably, projects of this scope and nature end up being more expensive than you plan. But you need to be realistic about what you can achieve. Don’t be afraid to design your dream car, but break it into rational stages. This is common practice, for example, with supercharger kits. Stage I kit might be just a basic supercharger with conservative boost level. Stage two might add an intercooler. Stage III might add fancy electronics for better AFR monitoring or inputs to an EFI system.

This principle can work on a micro or macro scale. Also, try to be logical about when stages need to occur. For example, when planning your low end build, you might as well do as much as you can while the block is out to avoid having to pull the engine again later; if buying those custom pistons means delaying that hot street cam until stage II because the cam can be done with the motor in the car, that’s probably a wise choice. Staging your plan will also help you prioritize your purchases and keep things reasonably on track schedule-wise.

Iterate & Refine
The task of planning out an entire project can be overwhelming, but if you plan methodically and systematically, one sub-system at a time, you can arrive at a well-conceived plan. You will need to consider each sub-system individually, but then assess how it impacts design choices in other areas. This may require you to iterate through each sub-system multiple times to arrive at a plan that is actually executable, not to mention achievable within your budget and time frame. As you work through the decisions and evaluation criteria, you may find a constraint (whether it be technical or budgetary) in a given sub-system that is interdependent on another sub-system which forces you to tweak and adjust the plan as you go. I’ll try point out dependencies between the sub-systems as we go.

It is worth noting that a forced induction car will require more variables to be considered than a normally aspirated car. The outline below is an attempt to be fairly comprehensive, so if you are tweaking an NA car, you can skip some of the material.

Outline of Major Sub-Systems
I am also presenting the sub-systems in the order in which I believe they should ideally be prioritized. This doesn’t mean that they would be executed in this order, but in terms of evaluating your prioritize, where you spend your time, money and effort, I believe that the sub-systems should be addressed in this order. However, there is no one right way to do this. Your objectives may be quite different than mine, which may lead you to prioritize differently. Also, you might wish to zero in on one specific aspect: if you only care about increasing your boost levels, and you don’t want to pull your motor, then you don’t need to worry about the block.

For each of the sub-systems, I have identified some (but perhaps not all) of the major decision points you will need to make, followed by a discussion of the evaluation criteria worth considering. Where possible, I use examples of product vendors, and I try to provide “street-pricing” information to give you an idea of what my research has uncovered. While not exactly presented in a decision tree format, if you walk through these sub-systems in order, it comes close to functioning that way. I also will be including cross-references, but only to some of the more useful threads in the forum: there are simply too many references for me to catalog them all in this thread (hint: Google is your friend). However, you may find my - automotive bookmarks file to be of interest. Finally, where applicable, I list known product vendors that may be of interest, although I am by no means professing or attempting to provide an exhaustive list.

Disclaimers
This “how-to” should be looked at as a decision support tool, NOT an exhaustive reference guide for building high performance cars. The commentary and evaluation criteria presented are NOT exhaustive, but should at least get you pointed in the right direction.

I am NOT an expert. But I am a trained, persistent and tireless researcher with strong background in analysis methodologies for decision support. The information contained below was gleaned from hours of reading, discussing, and surfing the web, which in NO WAY makes me an expert. I’m simply documenting what I believe I’ve learned in general, providing a specific application of that learning to a specific car, and providing a suggested framework for working through decisions and documenting design choices.

One of the most critical evaluation criteria for some of you may be if you plan to competitively track your car. Sanctioning bodies each have their own rules about what is permissible. Before you even think about a modification, make sure you understand the constraints imposed by the class you want to run in. I have no expertise in this area, and have made no effort to factor all of those variables into the content posted below.

Don’t take my word for it: validate what I write here by doing your own research

Any products I mention are not endorsements, merely examples.

The pricing mentioned aren’t pulled from my actual receipts or invoices, I am putting them down based on recollection. As such they should be looked at as “budgetary” numbers.

FUEL, IGNITION, & ENGINE MANAGEMENT
I have come to believe that engine management is the single most important decision that should guide the rest of your performance modifications. Converting to a modern engine management system is not without significant costs and effort, BUT it will deliver huge dividends, and will maximize any of the other investments you make. With the availability of Megasquirt, modern engine management is within the grasp of the hobbyist. For the serious gearhead or racer, commercial-off-the-shelf (COTS) products are available that rival or surpass factory provided systems in late model cars, including full blown sequential injection, traction control, braking control, launch control, etc. Your choices here will have a huge impact on everything else you do.

Fuel DeliveryChoices
Stick with the stock CIS system, convert to carbs, or convert to EFI

Evaluation Criteria
If you convert to EFI, evaluate Megasquirt vs. commercially available options. Identify sources for injector rail, inserts, and other ancillaries. Prices vary widely depending on the option you select. A bare bones basic Megasquirt setup might be achievable for ~$800 using junkyard parts, but a more realistic number including the more desirable options seems to be in the $1200 to $1500 range. Commercial options are pricey, and typically begin in the $2000 and go skyward from there.

If you stick with CIS, do you need to consider supplemental fueling due to higher compression pistons or forced induction? A supplementary electronic controller driving additional injectors can be had for ~$300, and piggy back MSD is also an option. Alternatively, you can stay strictly in the manual world and drive extra injectors with a rising rate fuel pressure regulator. It’s also worth considering a newly discovered alternative from Unwired Tools, the digital WUR, which runs between $400 to $700 depending on which model you need.

Carbs can provide improved response and drivability, and perhaps improved power output, but at the expense of fuel economy. Also, be aware that there are some single carb setups that would totally suck. The only way to go is dual carb, and there are at least two alternatives: the dual Webers and the dual Dellortos. The manifold are no longer made, nor are the Dellorto carbs. Also, the Dellortos are getting harder to find rebuild kits for, so the Webers may be the way to go. Best option is to source the parts used. Manifolds routinely sell used for ~$200, and the dual carbs will be anywhere from $300-$800 depending on condition. Rebuild kits will be in the range of $150, and you may have to fab up the linkage and vacuum assist stuff required to complete the conversion.

Evaluation Criteria
Aftermarket setups can be had in the $200-500 range depending on options. Adding a Ford EDIS setup to a Megasquirt (EFI or standalone) can be done for ~$150 for the parts (not including the MS controller), which provides a good upgrade path for later converting to full blown EFI. Alternatively, if all you care about is the ignition, you can run EDIS using another open source option called MegaJolt Lite Junior (a.k.a. MJLJ). If you do decide to go with a DIS setup, you’ll need to figure out a way to mount the trigger wheel, as well as a block off plate for the ignition distributor. There are also options for upgrading points-based or hall-effect-based ignition distributors to optically-triggered systems, most notably the Crane XR700 setup. Some respected forum members advocate against this approach, although my research on other forums has not turned up any disparaging information from people who have converted over to a Crane system.

Evaluation Criteria
Doing nothing on a maximum effort motor is foolish. A narrow band setup might be useful for very rough monitoring and adjustment on a street car. The serious tuner will consider adding a wide-band O2 setup for proper air fuel ratio monitoring. Plan on ~$300.

Evaluation Criteria
For certain applications, it may be desirable to replace the stock fuel tank with an aftermarket cell. Much of this decision will be based upon what type of fuel you plan to run (see Alternative Fuels section immediately following), as well as what type of venues you plan to run in. The advantages of running a fuel cell are lighter weight, improved safety (with proper installation), balancing weight bias by installing opposite the driver, alternative fuel compatibility, custom arrangement in the rear deck area to open up space for other components in the rear of the car (such as dry sump tanks, axle-driven alternators, rear mounted batteries, etc.).

Several of the popular aftermarket performance shops resell "fuel cells" for circle track or drag racing applications. Unfortunately, these inexpensive alternatives (i.e. <$300) are not FIA-certified, which is a requirement for both SCCA and NASA. If you have any plans to race competitively, you will need a FIA-certified cell, which drives the cost up. In addition to a proper bladder, tank, and metal enclosure (plus fittings), a FIA-certified cell will require a protective cage to be built. There are two primary manufacturers of FIA-certified cells I have been able to find: ATL and Fuel Safe. Prices will vary based upon capacity and construction. My research indicates that the least expensive FIA-certified cell from Fuel Safe can be had for less than $700 (depending on capacity), but other options like surge tanks, alcohol-safe materials, lightweight aluminum enclosure (vs. steel) can push the price a bit higher. Still, the cell itself can be had for less than $1000. The comparable ATL cells seem to be closer to the $1500 range.

As for fabricating a cage for the cell, a basic cage can be constructed with roughly 20 feet of 1" x 1" steel tubing. My installation used a little bit more due to the reinforcements we constructed in the rear hatch. I spent about $100 on materials (not including tools). Fabrication required roughly 8 hours, so fabrication expense will depend on your access to mig welding skills.

Evaluation Criteria
For high performance applications, particularly those involving forced induction or high compression engines, detonation avoidance becomes critical. As a result, standard pump gas may not be the best alternative. With the ever increasing price of pump gas, race fuel is also becoming much more expensive. An attractive alternative worth considering is E85 because it is inexpensive (at least within the U.S.) compared to conventional fuels, and it offers anywhere between 100-105 octane rating. However, E85 is not without its drawbacks.

The first thing to be aware of is that E85 is a formulation of conventional gasoline and Ethanol, an alcohol distilled primarily from corn (although other crops can also be used). Like any alcohol, Ethanol has corrosive characteristics which are exacerbated by the fact that it is also hydroscopic. This means that over extended periods of storage, Ethanol will attract water molecules, which can accelerate corrosion issues with untreated metals (like steel gas tanks and untreated aluminum in the fueling system). Furthermore, alcohol also causes certain rubbers and plastics to deteriorate more quickly. On cars manufactured after 1995, these concerns are generally moot because the OEM manufacturers were required to build systems that could accommodate gasoline blended with smaller percentages of Ethanol. As a result, newer cars generally speaking have "Ethanol safe" fuel systems (although they are NOT designed to run on E85...more on that below). However, pre-1995 cars, the fuel systems are highly likely to have many components that will be susceptible to exposure to Ethanol. On our cars, this includes the stock fuel tank (steel in the 2.0L platform, plastic in the 928's, 924S's, 944's and 968's), the stock fuel lines (both the hard lines and the flex lines), and the entire CIS system itself including the fuel distributor and WUR, and the fuel pump. All of these components should be addressed to be protected from Ethanol. This would include fuel rails, fuel lines, fuel pressure regulators, and fuel pumps adapted for use on EFI conversions as well. To be clear, Ethanol is not nearly as corrosive as pure alcohol or methanol, but it is corrosive enough that these components should not be ignored if you plan to convert over to E85 compatibility. The specific concerns for these components are rust for untreated steel-based components, corrosion for untreated aluminum-based components, and hardening, cracking, and loss of flexibility for rubber and plastic components (including the flexible nitrile lines used for our fuel injectors). Steel components should be powder coated inside and out; aluminum components can be anodized or powder coated inside and out; and rubbers and plastics should be replaced with suitable alcohol safe altneratives (teflon seems to be a common choice).

The second major concern with E85 is that Ethanol only produces about 70% of the energy per volume of gasoline. In practical terms, this has two effects: the first is that E85-based cars are generally about 25% less fuel efficient than their gasoline counter parts; the second is that the fuel delivery system must be able to accommodate a higher volume of fuel flow to achieve proper air fuel ratios. This situation is exacerbated by the fact that E85 formulations can reportedly vary anywhere from 70% to 85% Ethanol, which obviously has the potential for dramatically impacting fuel mixtures. The other problem arises with AFR monitors, which will see more oxygen in the exhaust stream and will consequently report incorrect mixtures. The solution to these problems is to run an ECU that has the ability to receive input from an E85 sensor. The job of this sensor is to tell the ECU how much ethanol is in a given volume of gas in real time so that the ECU can manage fuel mixture accordingly. The ECU will also have to be programmed to handle the skewed feedback from the O2 sensor.

The third concern is also related to the energy volume of E85: the standard 16 gallon fuel tank in a 924 will carry you fewer miles on E85. This factor should be considered if you are evaluating an aftermarket fuel cell. For example, a common practice on a dedicated track car is to reduce the fuel cell capacity to reduce weight. However, if you plan to run E85, it may not be prudent to run too small of a fuel cell, since you will probably only get 70-75% of the range available using pump gas.

The fourth concern is that the fuel pump arrangement must be addressed. It is not safe to use in-tank pumps with E85, so the in-tank pump must be removed. Also, the external pump must be replaced with a unit that is built from "Ethanol safe" materials, as well as having the additional capacity required to deliver more fuel due to E85's lower energy volume. At present, my research indicates the the Bosch 044 style pump is E85 safe (at least anecdotally), and also has the ability to deliver enough volume to support up to 600BHP applications (for conventional fuel...probably more like 450BHP for E85).

Notwithstanding, there are major benefits to E85. The first and most obvious is that E85 offers octane rating up to 105...that is near race fuel octane at normal pump gas prices! Furthermore, E85 runs much cooler and also much cleaner than conventional gasoline. Cooler is good from a thermal management perspective, as well as controlling detonation. Many anecdotal stories I have uncovered on E85 conversions also indicate virtually zero carbon buildup within the combustion chamber and related components (pistons, valves, plugs, etc.).

To net it out, from a total cost of ownership perspective, it is highly unlikely that converting a daily driver to E85 will result in a return on investment. By my rough calculations, it would probably take 10 years at prevailing fuel costs to pay for a properly executed conversion in a street driven car. The high cost is primarily due to converting the fuel system over to Ethanol safe materials, as well as the almost certain necessity of converting to EFI in order to properly adjust fuel mixtures. However, for high performance applications, E85 is a VERY attractive alternative, particularly if you are already planning to convert to EFI.

FORCED INDUCTION
For those thirsting for significant performance increases out of the 2.0L, the biggest bang-for-the-buck opportunity is in forced induction, particularly if you already have a 931. Converting an NA to forced induction is a bit trickier, so I’ll break this section down into those two categories.

Before we get to the good stuff, however, if you’re thinking about one of those “electric superchargers”, simply put, don’t waste another grey cell or microsecond. They are worthless, the automotive equivalent of snake oil. If you really insist on pursuing this avenue, just do a search on the forum for the many many heated discussions on this topic. Exhaust driven or belt driven is the only way you’re going to produce the CFM required to achieve forced induction. Period.

CONVERTING NA to FISuperchargerChoices
This is strictly DIY territory.

Evaluation Criteria
There are several examples here on the board of guys who have trod this path. Do some searching, and I’ll come back and post links later. But suffice it to say, there are NO bolt-on supercharger kits available for the 2.0L NA. I’m guessing that the price range starts at around $2000 and goes up from there. And word to the wise: if you stumble across the ESC944 and Hausbrauen-related posts, those guys were a bunch of scam artists, so any of that material should be viewed with a high degree of skepticism.

Evaluation Criteria
There are no bolt-on aftermarket kits currently being manufactured to my knowledge. So DIY really means do-it-yourself. This will include the whole shebang: developing a manifold, sourcing a turbo, finding room, modifying the intake and fuel system, etc. I’m sure it can be done, but given the cost and effort required, this is probably the path of MOST resistance

Adding a stock 931 turbo to an NA motor is almost in the realm of impossibility, unless you source a complete 931 engine and do a swap. Even then, you have to deal with wiring issues that are daunting to say the least, unless you decide to go EFI, which adds another order of magnitude to the cost & complexity.

The only real, viable option is to find a used BAE setup from an existing car. There are several threads on this board regarding the BAE kits and their viability. Do some searching, and I’ll come back and post links later. Cost is strictly what the market will bear for either a complete 931 engine or the BAE kit (if you can even find one).

There are a couple of notable examples of guys who have made their own manifolds, including DutchPug and Raceboy, who both routed the turbo over to the cold side (a la 951 configuration); bruce76-924, who used an upside-down install of an Audi manifold; and bass gt, who built a completely custom tubular exhaust header (all linked below).

Evaluation Criteria
The quickest, dirtiest, and cheapest path to more power from a stock 931 is to install a boost controller on the wastegate actuator line. DIY boost controllers can be made with basic hardware store parts, or you can purchase off-the-shelf manual versions (~$100) or sophisticated electronic versions (~$300). Anecdotal evidence suggests that boost can be safely increased to 10 psi, maybe as high as 12 psi. Any higher, and octane enrichment and thermal management measures must be added.

Boost EnhancerChoices
Lindsey Racing makes a boost enhancer that will work on the 931. There may be others.

Evaluation Criteria
This simple device prevents wastegate creep for a more controlled and direct application of boost. Doesn’t really increase power, but may improve the perception of power or the driving experience (~$100).

References
TBD

Suppliers
Lindsey Racing

Wastegate SpringChoices
Shimming, 1.0 bar, 1.1 bar

Evaluation Criteria
This approach might be worth considering on high-revving dedicated track cars expected to run on-boost for extended periods. Less applicable to a street car where boost levels will modulate a lot based on driving conditions, and where fine tuning and control is more desirable. Given the location of the 931 wastegate, frequent rebuilding or swapping of the spring is not a trivial task, although it can be aided by installing a flex coupler on the j-pipe. This technique is not advisable without intercooling or some other form of thermal management. Andial can provide 1.0 or 1.1 bar springs. There may be other sources, but I haven’t found them.

If you’re into this option, another alternative is to shim the stock spring (use search to find out how). I have also read some posts on modifying the wastegate housing such that the shim can be attached to a simple screw device that allows for adjustability. Cool concept, but again, given the location of the wastegate on the 931, this may not be the easiest or most elegant approach for increasing boost.

Evaluation Criteria
Not exactly a performance modification, but worth mentioning. If you’re planning to rebuild your 931 stock K26 unit, you should be aware that the 944 bearing housing is a direct bolt-in replacement that works with all of the stock 931 mounting points…with the added benefit of providing water cooling to the bearings, which are prone to failure in the 931. While synthetic oil seems to dramatically improve the K26 bearing life, this is such a low-cost viable option for improving durability that it shouldn’t be overlooked. Depending on the condition of the turbo, a stock K26 can be rebuilt for not much more than $300 at one of any number of reliable rebuilders. Sourcing the 944 bearing housing can be done through your rebuilder or via discussion forums, Craigs list or eBay, so price will be what the market will bear. I paid $65 for mine.

Evaluation Criteria
Generally speaking, the hot side of the stock 931 K26 unit is suitable for much higher HP numbers. However, if you’re targeting much beyond 250 BHP, the cold side will begin to run out of breath, especially a series II unit, which received a smaller wheel for faster spooling. Simply not enough CFM to support higher HP at peak efficiency. The solution is to source a K27 wheel, backing plate, and compressor housing, and have your rebuild shop mate it to the hot side. A new wheel can be had for ~$125 direct from Borg-Warner. Backing plates and compressor housings can be source from 944 units, so price will be what the market can bear, plus any machining costs from your rebuilder. I paid a grand total of $265 for a new wheel, a used backing plate, a used housing, and the machining to have mine modified.

Modern Aftermarket TurboChoices
Garrets seem to be the popular choice, but there may be others worth considering.

Evaluation Criteria
A modern ball-bearing and water-cooled turbo can be added, but at very significant cost. The biggest challenge is overcoming the stock exhaust manifold with its totally unique and otherwise non-standard 3-bolt mounting flange. The available options are to modify the stock manifold, build an adapter flange, or build a completely custom manifold. All of these have significant costs, probably in the thousands of dollars range unless you have access to welding equipment, materials, and are sufficiently skilled to do the fabrication yourself. The turbos themselves start around $1200, not including the modifications to the manifold and other custom fabrication.

THERMAL MANAGEMENT
As far as I know, the principles in this section apply equally as well to turbo and supercharged applications. So if either of those options is in your plans, read on. While NAs might gain some benefits to cooling the charge, the percentage of power increase is so negligible that it probably makes very little financial sense to consider using these techniques on an NA motor, with the possible exception of improving the coolant flow through the head and a more efficient radiator.

That said, on forced induction applications above 10-12 psi, detonation becomes a problem. Detonation can be controlled to an extent with higher octane, but the proper way to combat the real cause of detonation (heat) is to improve how the system removes heat from itself. There are several popular options to consider, but before we get into those, there are a number of factors that will influence your decision. Just remember that for ANY engine, heat is the enemy, so any thing you can do to remove or manage heat buildup will not only optimize performance, but will improve durability.

There is a lot of theory and conjecture about the pros and cons of air-to-air (A2A) vs. liquid-to-air (L2A) intercoolers. My research indicates that it boils down to this: unless space is a constraint (and in the case of the 931 that’s a big UNLESS which I will come back to later), A2A intercooling is better. Here are some of the reasons: fewer stages of heat exchange (one vs. two in L2A) means higher efficiency cooling can be more easily (i.e. more inexpensively) achieved; in most cases, A2A units are less susceptible to heat soak; A2A units are mechanically simple, and therefore cheaper to implement and maintain, as well as lighter. Now back to the big UNLESS: on a bone stock 931, there just aren’t very many easy-to-execute options for installing an A2A unit. I’ll highlight those constraints below, which you can use to determine if it’s worth the hassle to go the A2A route, or if L2A will be just fine. For most of us driving relatively stock, street-driven cars, L2A intercoolers may very well prove to be the more attractive option. You be the judge based on how much you are willing to modify the chassis, and what your ultimate goals are (street vs. race, target BHP, anticipated boost, etc.).

Evaluation Criteria
The first challenge is contending with the intake path on the stock 931. Simply put, about the only A2A option that doesn’t involve a tremendous amount of custom pipe work (see location topic below) is a stock 924 CGT top-mount IC. These are rare as hen’s teeth, commonly selling in used condition for $1500.

A popular option is to remove the badge panel and modify the underlying stock duct work to accommodate a 951 front-mount IC ($50 used up to $1000 for a Lindsey Racing Stage V FMIC). Less common is a conventional FMIC unit in front of the radiator and a/c condenser (off-the-shelf FMICs run between $300-$1000 depending on size and construction), or to relocate the battery to the rear and put a top-mount IC in the battery location (small TMIC cores can be had for ~$300, or you can source a used Ford T-bird or a Mazda RX-7 core, which could be adapted).

Virtually all of those options involve significant cosmetic surgery to the chassis (hood, nose panel, front valance, etc), which may not be desirable. On the other hand, if you’re building a race car, and cosmetic surgery is not a concern, by all means, find a way to implement A2A.

On my wide body project, a very nice alternative was the side-mount IC, located beneath and behind the passenger side headlight bucket. This option is really only viable with a fixed headlight conversion, and while you might be able to squeeze a small core in there with standard fenders, 944 or wide body fenders give you more room to work with. Of course there are other things you can do to mitigate the cosmetic constraints: if you choose to convert to EFI, a tremendous amount of room will be freed up by removing the CIS assembly; or you can do what some of us (myself included) have done and develop a custom intake manifold that is better positioned for a direct run from a FMIC outlet.

Evaluation Criteria
A properly executed L2A system will typically cost in the $1500 range. You have to source the L2A heat exchanger for the IC itself, a small radiator to remove heat from the coolant, a capable coolant pump, and possibly a reservoir. And that doesn’t include any of the plumbing. The big upside is that a L2A unit requires much smaller heat exchanger, and therefore can be implemented with little-to-no cosmetic surgery. There are a few styles to consider, including the conventional enclosed heat exchanger (usually rectangular), a barrel-style enclosed heat exchanger (such as those by PWR), or the so-called Thermal Hard Pipe from Lindsey Racing.

Your choice will largely be dictated by placement of the heat exchanger relative to the charge air path, and the choices you make to minimize bends and plumbing runs. You will also need to find a place to mount the secondary heat exchanger (basically, a small auxiliary radiator), but these are usually fairly small and can be tucked into the nose section, perhaps with some creative ducting. The enclosed heat exchangers often double as the reservoir, but you will also need to source a durable pump that can withstand the abuse of being run perpetually while the engine is on.

Evaluation Criteria
Intercooler Location: Where you locate your IC depends a lot on whether you go with A2A vs L2A, how much cosmetic surgery is acceptable to you, and your design choices for optimizing the charge air flow. Remember that the longer your runs are, the tighter and more numerous the bends, the more you will affect pressure loss (which in turn affects forced induction efficiency) as well as lag. Shorter runs, fewer bends, and gentler curves are all better, so do whatever you can to achieve that based on your location.

At first blush, the CGT looks like the ideal solution because it perches “on top” of the stock charge tube location, provides a direct run, etc. However, it is also directly above the exhaust manifold, and doesn’t exactly have the best air flow path into the hood scoop, thru the IC core, and out the bottom of the engine compartment. As a result, some people believe it actually acts as an “interheater” for stop-and-go driving conditions!

The battery tray location is a very attractive option for either top mount A2A or convetional L2A, as long as you don’t mind the cost and trouble of relocating the battery; there are a few examples of successful implementations here on the board. At least one board member has implemented a PWR barrel style L2A that required virtually no cosmetic surgery; his installation was aided by fixed headlight conversion, but he reports that it should be doable even with pop up lights if you loose the windshield solvent bottle and the headlight stop bracket on the passenger side.

Front mount locations can be tricky on stock cars because the IC can dramatically affect the performance of the stock radiator, leading to a ripple effect on the engine coolant heat management.

Evaluation Criteria
One very interesting method of thermal management is to inject a mix of water and methanol into the charge path. This technique has the dual benefit of cooling the charge air and acting as an octane boost. However, be aware that the venerable Corky Bell dismisses this technique as a band-aid for proper engine management. Nevertheless, several board members are using this technique with good results. It is mechanically simple, relatively cheap ($300-$800 depending on how much you DIY vs. buy off-the-shelf), and anecdotally appears to be effective for boost levels as high as 15-17 psi. I suspect that for many of us, adding a boost controller and a methanol injection kit would be the best bang-for-the-buck (and effort) available.

Evaluation Criteria
This technique involves spraying a consumable liquid medium (water or nitrous) onto a conventional A2A intercooler during on-boost driving to aid in intercooler efficiency. Simple and effective, it can be done for relatively modest investment. Probably overkill on a street-driven car, but desirable on a track car, the only down side I see to it is the consumable medium.

References
None known at this time, although Steve Bassington is planning an install for the 2008 season…

Suppliers
TBD

Improved Coolant FlowChoices
DIY

Evaluation Criteria
Steve Bassington has documented a method of improving the water flow through the stock 931 head. In effect, it is quite simply a larger capacity, custom-designed crossover pipe that replaces the stock item. The effect is that it equalizes flow from the front and back of the head, eliminating the apparent hot spot on cylinder 4 that seems to plague some 931s. Link to his post will be added soon. Be aware that on a stock car, this approach might need to be tweaked a bit to accommodate effective coolant flow to the heater core.

Evaluation Criteria
This technique is used all the time in the muscle car world to remove parasitic power loss from the crank, theoretically freeing up anywhere from 2-5 HP. Davies Craig makes a universal setup that is essentially bolt-in for the 924, although for ultimate installation, some modification of the water pump and t-stat housing may be desirable. This technique may not be suited for a street-driven car due to concerns over the lifespan and durability of the electric water pump for every day driving. The cost for this option is between $300 and $500 depending on how you choose to implement it.

Evaluation Criteria
There aren’t exactly any drop-in upgrades for the stock unit, but with a little creative grinding and light fabrication, a 924S, 944 or 951 radiator can offer improved efficiency over the stock unit for a price around $400 new. Alternatively, Ron Davis makes a nearly bolt-in high efficiency rad for the 924 for around $625 (see Rasta Monsta’s post). Any of the radiators (stock, 2.5L rads, or Ron Davis) will benefit from a modern, high efficiency fan such as those from Spal or Davies Craig (among others; ~$150). Anecdotally, the Performance Products / Automotion high-performance radiator is a piece of junk. There are also several speed shops and local radiator shops that can fabricate custom radiators to your specifications.

Evaluation Criteria
Simply put, (barring liquid-to-gas chemistry magic) there is no liquid on earth that transfers heat as efficiently as water. Not even glycol. Running a higher ratio of water:glycol will improve cooling, although it’s a generally accepted that glycol will help with corrosion resistance. So run more water, maybe 75%:25% (except during winter driving). Water wetter helps too. Cheap and effective.

Some of the big block straight line racer types will actually run a solid block. Unless your game is straight line short run racing, I doubt this option makes sense in a 924.

Evaluation Criteria
The stock 931 oil cooler does a decent job, but can be improved upon by adding a larger, more efficient modern aftermarket oil cooler (~$300 or so), as well as a remote mount filter. The stock 931 has a thermostatic control in the block adapter, but if you’re adding an oil cooler to a supercharged or NA engine, you’ll want to add that to your shopping list. These components are all relatively inexpensive, but a complete oil cooler setup can easily approach $600 by the time you source the block adapter, thermostat, remote filter mount, cooler itself, fittings, and steel braided line. It’s also worth noting that adding an oil cooler to an NA is a viable option for controlling engine and underhood temps.

Evaluation Criteria
Ceramic coating of headers or exhaust manifolds can be jobbed out for around $50-$75 per piece. This is absolutely worthwhile on a turbo to keep the exhaust temps up as they enter the turbine, improving the turbo’s efficiency. For supercharged and NA applications, the performance benefits are questionable and unquantified on the 924, but they will help maintain exhaust temps and velocity; more importantly, coatings or wraps will reduce under-hood temps which in the long run will contribute to improved durability of the overall engine. Some people have reported that wraps actually work better than ceramic coating at containing heat within the header, but this may be dependent on the heat range of the coating used. Do your own research and price comparison to determine your approach. Remember, heat is the enemy, so anything you can do to reduce it is good.

Evaluation CriteriaNOTE: This discussion is strictly experimental at this stage. There are no known installations at the time of this posting, and there are not a lot of documented results on other platforms.

Phenolic materials are manufactured composite resins with a fiber base, typically paper, fabric (cotton, linen, wool), or glass. Phenolic materials generally have low conductivity of both heat and electricity, high strength and distortion rejection characteristics, many are resistant to petroleum-based products, and most formulations are considered to by very machinable. A phenolic spacer is a technique used to create an insulating layer of plastic between the head and the intake manifold. Theoretically, the insulating spacer prevents heat from the head from being conducted into the intake manifold. Since 924/931 heads and intakes are aluminum, and aluminum is an excellent heat conductor, it seems logical that a phenolic spacer would be a good method of controlling, if not outright reducing, charge air temps.

There are currently no known suppliers of phenolic spacers specifically for the 924/944/968/928 series. However, phenolic materials can be ordered online from plastics suppliers. The material comes in a variety of grades, each with different heat characteristics, as well as different sizes, ranging from 12"x24" to 48"x48" to offcuts to custom cut sizes, and also in thicknesses from fractions of a millimeter to hundreds of millimeters. It seems feasible for the reasonably equipped hobbyist to be able to make his own spacer by using an intake manifold gasket as a template, and then using a jigsaw and drill press to duplicate the shape. Alternatively, it may be possible to work with a local machinist to fabricate a spacer by providing a template based on a gasket.

The ideal thickness is a minimum of .25" (roughly 5-6mm), and the ideal material appears to be a grade known as G10, which can withstand temps as high as 284°F / 140°C. Be forewarned that there may be fitment issues related to the various brackets that attach to the intake manifold, including the main support bracket on the underside of all 924 intakes, the charge tube fitment on a 931, and also the "endwrench" style fuel rail for EFI conversions.

BOTTOM END
The old saying is that there’s no replacement for displacement. The block is the foundation of the motor, and given the torque-challenged 2.0L 924 block, improvements can certainly be had in this area. Conventional techniques can be applied here, with some caveats as noted below.

Crank ModificationsChoices
Stroker, knife-edge, scrapers

Evaluation Criteria
Consider increasing the stroke by welding and offset grinding the rod journals. European Motorworks claims the stroke can be increased to as much as 98mm(!). However, be aware that any stroke increase will cause block interference, so you will either need to go with custom rods with a smaller profile, or you will need to remove material from the block and/or the stock rods. Increasing the stroke will also require pistons with a different pin height, so unless someone can find an off-the-shelf replacement piston with a suitable pin-height-and-rod configuration, custom slugs will be required. You can also consider knife edging and lightening the crank, as well as adding a crank scraper to reduce windage. A weld-and-grind stroker crank can be had for ~$450. Knife edging and other crank services are generally available at your local machine shop for prevailing rates.

Evaluation Criteria
The stock rods are identical on NA and Turbo applications, and are quite sufficient for high power levels. However, if you stroke the crank, you may need aftermarket rods that have a lower profile on the big end. The other big advantage of aftermarket rods is lighter weight. Pauter makes light-weight chrome-moly duplicates of the stock rods (~$1200 per set), and Crower makes custom light-weight forged moly rods that will accommodate up to ~91mm of increased stroke (~$800 per set).

It’s also worth mentioning that if you are considering crank modifications and custom pistons, there is no reason you can’t look at existing off-the-shelf rods from other platforms that might be cheaply and readily available. The factors to evaluate would be having the crank rod journals ground and sized to match the rod big ends, having the piston pin height set to match the rod length, and having the piston pin diameter set to match the rod little end. This might be an effective way of reducing the overall cost of a stroker, considering that a stroker crank and custom pistons usually go hand-in-hand. I don’t have any specific suggestions of suitable rods to consider, and I’m not aware of anyone having done this, but it is a viable option given enough time and research.

Evaluation Criteria
OEM replacements for the rod bolts are NLA. Fortunately, in 2008, this community was successful in commissioning an aftermarket replacement for the stock units, which are now a catalog item from ARP. The part number is AR336. These units are made from a higher grade material (ARP2000) than OEM replacement because several members are running higher output forced induction, and are rated at 220 ksi. While these are now a catalog item, they are not stocked. EBS participated in the group buy organized here, and may have a small stock available. Otherwise, another group buy will have to be organized through in order to procure these.

Evaluation Criteria
Decisions on the crank and rod will impact your choice for pistons. However, even if you don’t modify the crank and rods, you might consider replacing you stock slugs with Euro spec or custom slugs in order to achieve higher compression, or to make your 931 a non-interference design. There are also VW flattops to consider. The other common practice that applies is to increase the bore size to achieve more displacement. Stock is 86.5mm. 87mm and 87.5mm were the factory-available oversizes. Anything beyond that and you would be well advised to have the block x-rayed to make sure there is enough material between the cylinders. Custom pistons can be had for between $125-$200 per slug, depending on options such as ceramic and Teflon coatings. For high boost applications, these coatings are a good idea, though perhaps not mandatory.

Evaluation Criteria
Your piston choice will determine what to do with the cylinder bores. See constraints noted above. Another alternative commonly used to increase compression is to simply mill the block deck (leaving the crank-rod-pistons alone). However, be aware that removing too much material will affect your cam timing, and you will have to make other compensating adjustments (such as a vernier style cam pulley). At a minimum, the block deck should be clean cut and checked for true. Align-honing of the mains is also advisable. If you plan to run any form of forced induction beyond stock boost levels (~6 psi in US spec, ~9 psi in Euro spec), o-ringing the block is a must. Most of these services can be performed by your local machine shop for prevailing rates. Budget at least $200-$700 depending on what you select.

Evaluation Criteria
Unlike muscle cars, there are no high-volume replacement oil pumps available, at least that I’ve been able to find. In order of expense (lowest to highest), sump improvements can be made by adding an oil accumulator (such as those from Canton or Moroso, ~$400 for complete kit), a vacuum assist pump (such as those from GZ Motorsports, ~$900 for complete kit), or a full blown dry sump (such as the bolt-on setup from Pace Products, ~$1600). The muscle-car practice of installing a windage tray or oil pan baffles is generally not applicable to our cars, but a crank scraper (such as those available from Ishihara-Johnson, ~$100) are an inexpensive alternative.

Evaluation Criteria
Rotating mass can be reduced by either lightening the stock unit (which some argue is dangerous) or by installing an aluminum off-the-shelf unit from Fidanza (~$350). However, despite what many of the aftermarket catalog resellers depict, we have recently learned that the Fidanza unit (part number 199241) is NOT available for the NA cars, and apparently, is NOT compatible with the 931 either. It is possible that the 199241 item was made based on a late Series 2 931. There were three different flywheels produced for the 931, the first two having identical pressure plate bolt pattern, profile, and center bearing diameter, the late version being setup for a rubber-centered clutch. At the time of this writing, we are still in the dark as to origins of the 199241 part. Fidanza claims they have hundreds of these in the field, but these units clearly have a different support bearing diameter in the center, and according to two other independent sources, a different bolt pattern for the pressure plate. Spec also makes an aluminum flywheel for the turbo (~$600), but there are no reports of anyone using one so fitment is still an unknown.

Evaluation Criteria
4 cylinder motors are different beasts than V8s, and the 924 motor is apparently particularly prone to vibration. There is no vibration damper, so external balancing is not really an option. For a smoother running engine, internal balancing is highly recommended, especially if any of the above modifications are planned. Budget around ~$600 or more.

HEAD
The next place most folks go to improve performance is the head. Unfortunately, the 924 head is the “Achilles heel” of our platform. It is widely reputed to be the major barrier that prevents all of the usual bolt-on improvements commonly used on other platforms from delivering any measurable improvements (such as cold air intakes, porting, polishing, etc.).

The turbo head is different than the NA, and is only marginally better. Putting a turbo head on an NA can be done, but complicates exhaust plumbing because the manifold ports and bolt patterns are different, and IMO is not worth the trouble. Jon Mitchell in the UK has purportedly developed some modifications for the NA head, but the details are not publicly available at the time of this writing. About the only commercially viable option I know of is the so-called “big-valve” head, available from European Motorworks (formerly Eurorace). Here is the required reading on the short-comings of the 924 head:
Heavy Breather

CamsChoices
Stay with stock, regrind, aftermarket billet grinds

Evaluation Criteria
There are fairly good options available for cams, from reground cams to complete custom grinds on new billet stock, which can be had for reasonable money. The most important evaluation criterion is where you want the power band. For street use, a stock grind or hot street grind is widely considered to be the best option. For race use, it depends on your venue and where you intend to spend most of your time RPM-wise. Most of the cam grinders offer street, stage I (hot street), stage II and stage III grinds. While there are slightly different profiles available for NA and Turbo cams, the same principals apply. Basically, if you are not building a full-time track car, go with either a stock or hot street profile. Costs are typically less than $400. Be aware that new cams require new lifters (see below).

Evaluation Criteria
There are four known options: new units from Porsche in Germany for ~$80 each, not including the adjuster screws; solid lifters from Piper for ~$450 per set; solid lifters from Arrow Precision for ~800 per set; or take your chances with used units. Be aware that solid lifters require the use of shims (instead of the adjuster screws), and are a royal PITA to install on your own. Probably best left to your machine shop or head shop.

Evaluation Criteria
For forced induction applications, it is fairly common belief that stock valve sizes are suitable, although a slightly larger intake might be helpful. For NA applications, breathing can be improved by increasing the intake and exhaust valve sizes, the most common approach being to send the head to European Motorworks. Their standard service includes replacing the valve guides, installing new valves with smaller (8mm) stems, new keepers, and new springs. According to Jorge, the lift on most 924 cam grinds is relatively modest and therefore, uprated valve springs are really unnecessary. Cost for big valve head is usually quoted around ~$1100.

One member has identified a possible option for sourcing completely custom valves, but this option is so far untested within the 924 2.0L universe, at least as far as I know.

Evaluation Criteria
You can stay with the stock unit unless you do something exotic to the head. If you shave the head or block in order to increase compression, you will probably need to get a vernier style (a.k.a. adjustable) Cam gear, which typically allow for +/-3° of static adjustment. These are routinely available for ~$100-$200. Alternatively, if you are lucky enough to locate and purchase a Franco variable Cam Gear (Franco Industries is now defunct), these are most useful on a forced induction application where the cam timing is automatically retarded up to 10° from a baseline timing setting, based on RPM using a centrifugal weight system within the cam gear housing. Three models were available from Franco: +10°/-0°, +8°/-2°, +7°/-3°, so be sure you know which one you’re after. The +7°/-3° version is probably the best option for forced induction. These may have some benefit on carbureted motors, but I have no practical experience. The Franco gears were originally priced at $350; current pricing is subject to what the market will bear.

Evaluation Criteria
Raceware head studs are highly recommended for any high-compression or forced induction applications. ARP head studs made for the Ford Cosworth turbo engines are a direct-fit replacement for the stock units. The ARP part number is 251-4701, and these are readily available from many of the online race shops (such as Summit Racing). There is also a lot of debate about replacing the stock head bolts any time they are untorqued (e.g. during a top-end rebuild) due to concerns over the effect of stretching and restretching on the tensile strength of the bolt. FSM says to replace with new. However, several members have successfully reused stock head bolts for street applications.

Evaluation Criteria
Stock head gaskets are widely regarded as suitable for almost all applications (~$75). For higher compression or forced induction beyond stock levels, block o-ringing is recommended. Generally speaking, copper head gaskets can be had, but they tend to be pricey (~$110 to $250 or more), and are not recommended because they cause electrolysis to occur between the aluminum head and the iron block. This in turn causes corrosion and damage to the head. There have also been multiple reports of sealing issues.

Another option is a composite gasket from Lubbock. These use a plastic type composite material with copper crush rings in the cylinder bores that are purportedly the equivalent of o-ringing. They can be had for about $100. I have one, but it isn’t installed, and I’m not aware of anyone who has tested or run one of these on a 924/931. The research I've conducted seems to indicate that these are essentially equivalent to the OEM units, and NOT an upgrade.

Until recently, multi-layer steel (MLS) head gaskets were not available for the 924 2.0L platform. In Feb 2009, this community was successful in commissioning Cometic to produce a custom MLS gasket. Cometic's MLS gaskets provide superior sealing due to embossments around all of the ports, as well as the Viton-coated stainless steel, which is laminated into 3- or 5-layer gaskets depending on thickness. The customer can specify thicknesses from .027" to .140" in order to compensate for over-milled blocks or heads, and to achieve the precise compression ratio desired. List price is expected to be around $110. While this part will become a catalog item, at the time of this writing, EBS declined to participate in the group buy, and there are no other retailers that are stocking the item, so future purchases will likely require another group buy. NOTE: Do NOT o-ring your block or head if you intend to run Cometic MLS gasket. According to the manufacturer, the o-ring will interfere with the special embossments they build into the gasket, which are designed to provide superior sealing.

Evaluation Criteria
As noted above, the actual design of the head is the problem, and no amount of porting or polishing seems to have any measurable effect. The consensus view here on the board seems to be that basic port matching is sufficient. In fact, some theorize that a mirror polish actually harms fuel atomization and causes a loss of low-end torque.

INTAKE
Due to the head issues discussed above, major intake modifications will yield very little in terms of direct contribution to power. They can, however, contribute to the overall design of your system as detailed below.

Cold Air IntakeChoices
DIY, aftermarket cone filters

Evaluation Criteria
As noted above, the restrictive head of the 924 means that there is absolutely NOTHING to be gained by installing so-called cold-air intakes or aftermarket cone filters (like K&N elements). There have been many debates about seat-of-the-pants results, but nothing substantiated. Given what we know about the head, the only reason I have converted to a cone filter on one of my 931s is due to the lower cost of using a reusable filter over the 931 stock cartridge, which seems to be getting harder to find and more expensive. That said, if other measures are taken to improve the flow of the head, perhaps helping the engine find colder air to begin with might theoretically provide some benefit, but it is still doubtful that it would be measurable on the dyno.

Evaluation Criteria
For NA cars, you can switch to a larger Audi TB (a nearly bolt-on “upgrade”), or if you can find one, a rare Weber redline TB. Throttle body “upgrades” actually do very little for raw HP output, but they do provide noticeable improvements in throttle response on the NA, an upgrade well worth considering given the modest cost (usually less than $100). A forum member recently posted a thread providing a somewhat scientific comparison of the various TB characteristics, which is worth a read.

For 931s, it is fairly common to convert the ovoid Series I TB and charge tube to the Series II versions which are round. This is usually desirable not for performance reasons but to improve sealing of the charge air path, particularly when increasing boost levels.

The ultimate option, particularly if you’re considering converting to EFI, is to go with independent TBs, but this approach requires significant investment (perhaps in the $2000 range minimum), as well as a custom manifold design. There are some notable successful examples here on the board, so if you’re serious about performance and engine management, this option is the optimal choice.

Evaluation Criteria
If you’re converting to an ITB setup, a custom manifold is unavoidable. If you’re installing a FMIC, a custom manifold is highly desirable. Be aware that many of these designs require modifying the mounting of the alternator and stock air-oil separator. Brake booster and coolant reservoir locations are also potential interference points. In my case, I developed a diffuser style secondary plenum out of a tube of aluminum stock that was welded onto the stock 931 manifold, preserving runner length, and allowing all other components to remain in stock locations, while positioning the TB and intake path directly in line with the outlet from a 951 FMIC. If you can weld aluminum, a custom manifold can be produced fairly inexpensively, otherwise, you can figure on about 10 to 20 hours of a welder’s time at going rates.

EXHAUST
The mythical 10% BHP gain from free flowing exhaust, like other common techniques on other cars, simply doesn’t apply to the 924. If you don’t modify the head, there’s no point going crazy on the exhaust. Furthermore, increasing the diameter of the exhaust system may actually worsen the situation: backpressure is generally considered bad for high RPM power output, but it helps with low end torque production, which as we’ve seen on the 2.0L motor is an inherent weakness. Consequently you would be well advised to avoid the conventional approach to exhaust systems that you may have read about delivering cheap bolt-on HP. Nevertheless, if you do address the head, improvements to the exhaust fall into the category of “it probably won’t hurt”, within certain limits.

HeadersChoices
Aftermarket, custom

Evaluation Criteria
Applies to NA applications only. MSDS, Stahl and Bursch all make decent headers that bolt on and work with the rest of an otherwise stock exhaust system. These can run upwards to $500 new, or can be acquired in used condition for ~$200 or so. Performance increase is questionable, at best probably no more than 10BHP contribution, but they purportedly add to a good sounding system, which may be enough satisfaction to warrant the expense. When combined with a big-valve head, results might be better, but I have yet to see any dyno-substantiated numbers.

Evaluation Criteria
If you’re fortunate enough to live in a region where older cars are emission-exempt (or you’re building a track-only car) – and if you’re not militant about pollution controls on cars – you can free up some exhaust restrictions by replacing your old, plugged up, non-functioning cat with a straight pipe. These can be made at your local muffler shop for probably less than $100. Alternatively, if you want to go to the trouble, and the exterior is not too rusty, you can simply gut the internals out of the stock unit. Or you can buy one of those fancy free-flow cats that routinely sell for $500+.

Evaluation Criteria
Depending on what your plans are to improve low-end torque elsewhere in your build, you might consider a 3” exhaust from the cat back to provide freer flow. I’ve not seen any documented dyno results that substantiate power increases, but it’s reasonable to assume that if you’ve supplemented torque elsewhere, you won’t have to worry too much about reducing backpressure with a freer flowing exhaust.

Coatings & Wraps
See this topic discussed in the Thermal Management section.

Mufflers & ResonatorsChoices
Stock, aftermarket

Evaluation Criteria
Again, I suspect there is not much performance increase to be gained in this department, so select and tune to your audio tastes! There are some interesting discussion here about different combinations of mufflers, resonators, glass packs, etc. so search is your friend.

931-SpecificEvaluation Criteria
As noted elsewhere, the exhaust manifold is a constraint on the 931 due to the turbine mounting flange, so unless you’re planning to modify it or go to the expense of creating your own custom manifold, you’re pretty much stuck with the stock unit. There are no aftermarket options. There are a few noteworthy examples worth studying on this board.

The cat-back portion of the 931 exhaust is reputedly identical to 944 cat back systems, so any of the off-the-shelf options for 944s are available for your consideration.

The wastegate exhaust pipe (the one that rejoins the main exhaust just aft of the cat) is a notable impediment to free exhaust flow, so if are willing and able to overlook the emissions issues, running a separate pipe or rejoining just before the final muffler may be worth considering, particularly on a high-ouput dedicated track car.

CLUTCH & TRANSMISSION
There is a ton of info on the gearboxes for our cars, so I don’t want to repeat it all here. If you’re unfamiliar with the various styles of boxes and the model years in which they came, do your research using the search feature and the FAQ, then come back here for a quick run-thru of some evaluation criteria.

Evaluation Criteria
The first factor you will need to evaluate when considering an upgraded gearbox is the size of the input shaft on your stock platform. All normally aspirated 2.0L 924's (a.k.a. NA) came with a 21mm input shaft & driveshafts and a cable driven clutch. All turbo-charged 2.0L 924's (a.k.a. 931) came with a 25mm input shaft & driveshaft with a hydraulic clutch. While it is not impossible to convert a cable driven setup to a hydraulic setup, I am not going to cover that conversion in detail for the time being.

If you have a 1981-82 NA 924, there isn't really an effective upgrade path without also converting to a hydraulic clutch. If you have a 1979 NA, you will need to find the gearbox including the mounts, the torque tube, and the complete shifting mechanism from the lever in the cabin all the way back to the linkage at the gearbox. You will also need to either remove the mounting ears from the snailshell torsion carrier, or you will need to swap in an audi-based torsion carrier (which might make sense if you are also considering suspension upgrades, such as a five-lug conversion or upgraded torsion bars...see the Suspension section below). If you have a pre-1979 NA, you will need to find the gearbox the shift rod. The torque tube, transaxle mounts, shift linkage, and shift lever are all the same. The shift rod itself is a bit shorter on the 5-speed, so you will need it. The 21mm Audi 5-speeds are generally widely available and can routinely be gotten in good condition for less than $300 from donor cars, salvage yards, eBay and Craigslist.

If you have a 931 of any variety, your car can be more easily upgraded because the 25mm input shaft and hydraulic clutch setup will be compatible with a wider range of later configurations. If you have a G31 snailshell, you should first check to see if you have an LSD by checking the serial number (which will end with "/12") or by checking to see if the rear wheels rotate in the same direction while jacked off of the ground. If so, that gearbox is worth keeping. If you have a snailshell, it probably needs rebuilding due to the weak 2nd gear synchros and dogteeth, and the shifter lever bushings probably also need replacing. If you’re lucky enough to find a rebuilt unit, it’s worth keeping for the shift pattern (particularly in a track car), as well as the upgradeability of the diff (see below). Otherwise, the $1500 rebuild cost will probably convince you to examine other alternatives. Be aware that the torsion carrier for snailshell-equipped cars is different from audi-equipped cars, so doing a swap is not exactly trivial…doable, but involved. Well documented elsewhere, so use the search feature.

Otherwise, the G31 (like the early 4-speeds and G16's) was notorious for having bad first and second gear synchros. In actuality, this is not anything unusual or peculiar to those gearboxes, as most old manual transmissions will suffer worn 1st and 2nd gears simply because of normal driving wear and tear. The problem with the G31 in particular is that the 1st and 2nd gear dogteeth and synchros are apparently unique to that car, and are very expensive to obtain if they can even be found. If you have this problem, you will either need to adjust your driving style in low gears, or switch over to a later Audi-based gearbox. To do so, you will need the transaxle itself plus it's mounts, the torque tube, and the complete shifting mechanism from the lever in the cabin all the way back to the linkage at the gearbox. As described above, you will also need to either modify or change the torsion carrier.

If you already have an Audi 5-speed unit, it’s a good reliable box that has some upgradeability. Be mindful, however, of the two sizes of input shafts / drive shafts. If you’re planning to create a torque-monster out of an NA by stroking it or supercharging it, you might want to consider upgrading to a 931-based torque tube / Audi box. This will require converting to the hydraulic clutch system, which is not covered in detail here.

If you are looking at upgrading your 931 to any of the Audi-based options, it should pretty much a bolt-in operation. Any of the 81-82 931 gearboxes, and any of the 924S/944/951 gearboxes are pretty much direct bolt in with the parts described above. However, be aware of the tall 5th gear in the post-83 gearboxes, which may make them unsuitable for a 924/931. This will depend somewhat on your intended driving characteristics, but don’t just jump all over that 951 LSD box without considering whether the gear ratio is really suitable for your needs. Some of the 944/951 gearboxes that pop up on Rennlist, Pelican and eBay have been specifically modified with a shorter 5th gear...these are desirable, and usually the price is reflective of that. Also, if you can find an LSD from a 944/951 unit, it can be mated to an early 924 Audi 5-speed, but this requires specialized tools and skills, and is not an inexpensive proposition. The 944/951 LSD-equipped gearboxes routinely sell for $800-$1600 depending on condition.

It's worth noting that the G31 with LSD option is a viable "retrograde" path for any 931 equipped with an Audi-based 5 speed. Because the original Series 2 931's did not come with an LSD option, a G31 with LSD is a very attractive option, particularly if you can find a unit that has already been rebuilt or if you can adapt your driving style to accommodate slow 2nd gear shifts. Many drivers consider the G31's Getrag or "dogleg" shift pattern to be more performance oriented and prefer it over the Audi's standard H-pattern. The retrograde is not trivial, however, as it absolutely requires the G31 torsion carrier which has the transaxle mounting ears, unlike the torsion carrier on the later Audi-equipped cars. You will need the gearbox, the torsion carrier, the transaxle mounts, the torque tube, and the complete shifting mechanism from the lever in the cabin all the way back to the linkage at the gearbox. If not for the scarcity of the aforementioned synchros and dogteeth, this option would undoubtedly be more popular with 931, 944, and 951 owners alike.

The ultimate gearbox for these cars is the 968 6-speed with LSD. The are extremely expensive (north of $3000 typically) and also require significant custom fabrication for mounting in a 924/931, so this swap is not for the faint of heart – or the thin-walleted

Finally, be sure to double check this information against that contained in the Garage FAQ (see first link below). That reference should be considered authoritative at this point.

Evaluation Criteria
Most 924s and 931s came from the factory with open diffs. A few of the snailshell equipped 931s had a factory-installed LSD, which by all accounts is identical to the 915 LSD unit (and possibly the 930 as well). Consequently, if you’re not one of the few who have found a factory LSD-equipped snail shell, you can consider modifying a good condition snailshell, by sourcing a 915 LSD. I have not priced this option so I can’t comment on the approximate cost. In any event, be aware, this is NOT a DIY project. You will need to find a competent Porsche gear box specialist (hint: not necessarily at the dealer…plug into your PCA network). Expect 6 to 10 hours of mechanics time from a skilled practitioner at whatever rates he can command. Nonetheless, if you doubt the merits of LSD, read this article: The Limited Slip Differential

For the Audi based units, you have a few options: install a 951 LSD, provided the gearing ratios don’t conflict with your planned driving characteristics. A complete 951 gearbox equipped with LSD will routinely sell for $1200-$1500. Alternatively, you can consider commercially available options. By far, the most affordable of these is the Phantom Grip, although again this is not DIY installation territory. On the higher end, for serious tuners only, are the Quaife and Guard options (falling into the “if you have to ask you can’t afford it category”).

There are at least two guys on the board who have adapted an Audi V8 TORSEN diff to work in a 924. However, the TORSEN style limited slip is really only suitable for street use.

Evaluation Criteria
Considering the 931 has a completely different clutch setup than the NA, for anything above 160 BHP, you would probably be well served to consider a performance clutch suited to your targeted HP range and driving use.

For street use, SPEC makes a couple of different stages that are definitely worth considering. For race use, they have even more aggressive clutch packages. Depending on your selection, you can expect to pay a minimum of $600 for an aftermarket performance clutch kit, upwards to $1000. General consensus around here is that the Centerforce products are not up to snuff for track use. YMMV.

For 931 owners, the 915 pressure plate provides double the clamping power of the stock 931 unit, and can consequently be combined with any number of clutch discs (from a stock Sachs unit all the way up to a SPEC Stage V) to achieve a result targeted for your application. Even more interesting is that the 915 pressure plate came in three versions: all cast iron (same weight as stock 931), hybrid aluminum+cast (lighter) and all aluminum. The all aluminum version apparently requires some modifications to accept the 931’s starter ring gear, but the other two versions are apparently direct bolt-ons. While they’re not cheap new, they’re really not that much more expensive than a new stock 931 pressure plate, so worthwhile in any event.

Evaluation Criteria
If you are replacing your stock engine mounts with performance oriented versions, it is also worthwhile to consider doing an equivalent upgrade on the gearbox end. For cars with Audi-based gearboxes, there are a variety of solid transaxle mounts available from the usual retailers serving the 944/951 community, which will be completely compatible with the 924/931 Audi-based gearbox. For G16/G31-equipped cars, the only option I am aware of at this time is DIY polyurethane knock-offs of the OEM units.

Evaluation Criteria
There are a handful of options available on the open market for the 944, which should all be compatible with any of the 924 Audi-based gearboxes. There are two styles of short shift kits available on the aftermarket: one style modifies the linkage at gearbox, the other takes a more conventional approach and changes the pivot points of the shift lever.

As for the snailshell's, the only available short shifter I am aware of is the one that Lizard was producing as one-offs a while back. I have one in my 941, and with Lizard's permission, I would be willing to share the dimensions with others for duplication, unless he is interested in producing more. Please contact him FIRST before asking me.

Evaluation Criteria
The shift linkage setup for the snailshell gearboxes consist of an overly complicated dual shift rod setup. The primary shift rod attaches at the gearbox input selector shaft using a universal joint common to some of the early 911s and 914s. The bushings at this u-joint are prone to developing slop. This issue is amply documented here on the board. The bushings are available in polyurethane from several of the usual resellers, including Pelican and Paragon.

In addition to the primary shift rod, there is also a secondary "guide tube" that attaches to a ball socket at the rear of the torque tube, and at the opposite end, provides the lower pivot point for the shift lever, and has a sliding insert on the end of the guide tube that attaches to a bushing that bolts to the torque tube tunnel. This unnecessarily complicated setup can be converted into a single shift rod setup by borrowing components from the later Audi-style shift mechanism. In short, you will need the saddle, ball socket, shift lever, and primary shift rod from an Audi setup. The saddle is easy enough to affix to the snailshell torque tube, but you will also require some additional components to adapt this setup to the snailshell. See the reference link below for a detailed How-to.

If you do not want to convert to the Audi-style shifter, you can also improve the end of the secondary guide tube, further eliminating potential slop in the shifting, by removing the relatively expensive Porsche-specific "angular joint" (i.e. the ball socket) at the end of the guide tube, and also cutting off the ball socket at the end of the torque tube. These can be replaced with standard off-the-shelf bolt and rod end components that are fairly easy to source and adapt to the torque tube with hand tools (hacksaw and drill). Again, see references below for a link to one example of this conversion.

SUSPENSION, BRAKES, WHEELS & TIRES
There is an almost overwhelming array of choices for upgrading the performance of your 924/931 suspension. And for good reason. The 924 handles amazingly well even in its most basic, factory-spec earliest stock form. Considering the limitations of the 2.0L engine, and looking at the range of suspension options that Porsche introduced to the same basic platform all the way to its zenith with the 968, it becomes clear why improving the suspension is considered by many to be the wisest investment on the 924.

Many of the improvements are possible by simply sourcing good used parts from later models, and replacing aging, worn wear components such as pads, rotors and bushings with OEM replacements. This approach can dramatically improve the handling characteristics – for less than $1000 – which ultimately translates into a faster car (provided you aren’t trying to build a 10-second quarter miler!). On the other hand, I was surprised to learn that the available alternatives for upgrading suspension performance can EASILY equal or eclipse the budget for engine work!

This topic area is by far the one in which I personally have the least amount of expertise. There are plenty of guys on the board that are much more versed in the art and science of suspension tuning, so we’ll need to rely on their assistance to supplement and validate from real world experience what I am presenting here.

Evaluation Criteria
The very first place to start with your suspension is to determine which 5-lug configuration you want to use. While the original 4-lug drum brake setup is perfectly fine even for spirited street driving, upgrading to a later 5-lug configuration is a no-brainer for anyone interested in serious performance handling. The good news is that all of the major non-wear components can be sourced from any number of available donors throughout the entire 924/931/944/951/968 family.

The exact parts required will vary depending on which platform(s) you choose as your donor, as well as which platform and gearbox you plan to swap onto. While it may be tempting to simply identify a single donor car and source everything from it, given the wide range of options Porsche introduced over the 14+ years of development on this platform, there is absolutely no reason you can’t mix and match components from a variety of model years and options to come up with an optimum configuration. There are many possible combinations that can work, and there are definitely some caveats to be aware of, so I will try to identify most of the major dependencies and constraints I have uncovered.

For starters, and to keep things simple, if your ONLY objective is to simply find the easiest path to moving from 4-lug to 5-lug, then find a 1985.5 or earlier 944, grab the entire suspension, and stop reading right here. It won’t be the best performing system you could build from available OEM parts, but it will be MUCH better than the 4-lug system you’re replacing, and you will be guaranteed an essentially bolt-on upgrade.

If you’re more adventurous, and very meticulous and careful in your planning, there are many more options for you to consider. The somewhat confusing part is that any combination of spindles, hubs, calipers, a-arms, trailing arms, etc. can be assembled from a variety of donors. This provides a great deal of flexibility in sourcing parts, but can also be confusing when trying to figure out which wear items (i.e. rotors, pads and bushings) to order new. Be mindful of the various styles and sizes of rotors available depending on the hub and caliper configuration you select.

If you’re still reading, I’m assuming you’re interested in assembling the best possible system from all of the available parts. Again, it’s tempting to think about finding a late model 951 and just lifting everything wholesale from it (and that can certainly be done), but there are a number of issues that you will have to contend with on your early platform taking that approach. Specifically, there are three super-critical aspects to consider, and they are all related to variations in the front spindles:

Do you care about maintaining your stock mechanical speedometer & cable, or is converting to a wheel speed sensor and an electronic speedometer an option?

Do you care about maintaining your stock strut inserts? This is particularly relevant if you've already invested money in upgraded front struts and springs for an early platform, but less important if you’re starting from scratch.

What brake calipers and rotors do you want to use?

There are a lot of variables to consider, so I'll try to walk through the decisions sequentially.

Critical Decision 1: Mechanical Speedometer
If you are unable or unwilling to convert to an electronic speedometer and wheel speed sensor, you have two choices for front spindles:

Use spindles from one of the following platforms: 1980 onward 924/931 with 5-lug or M471 option, 1983-1985.5 944, or 1987-1988 924S

Any spindle from any other 944 or 951 will require you to drill the LHS for the mechanical speedometer cable, and will also require you to switch strut housings. This should be doable by a competent machine shop, but I have not priced this out. It may also compromise the integrity of the spindle.

Critical Decision 2: Front Strut Housing
If you are unable or unwilling to change your front strut housings, you MUST source your spindles from one of the following platforms: 1980 onward 924/931 with 5-lug or M471 option, 1983-1985.5 944, or 1987-1988 924S. The upper strut mounting is universal, but the primary difference between pre-86 and 86-onward strut housings is the bolt pattern and flange width where the strut housing attaches to the spindle.

Critical Decision 3: Brake Calipers
If you want to run the better Brembo four-piston fixed calipers, you have three options:

Source the 1986 951 spindles, which are compatible with the early style hubs but do NOT have the speedometer cable hole and have the late style strut housing bolt pattern.

Source spindles from a post-1987 944 with M030 options or a post-1987 951, which will also require the hubs and front strut housings and do NOT have the speedometer cable hole

All of the other spindles have the SAME bolt pattern to accommodate the floating single-piston calipers, which can be adapted to accept the better fixed four-piston calipers by using adapter brackets from Lindsey Racing, VDub Engineering or Rennbay (and possibly others)

The point being here, if you want Brembos, but you selected an early spindle due to a constraint in Decision 1 or Decision 2, you can still upgrade using the adapter brackets. The drawback is that you will have the earlier style hub that are harder to service and have a smaller (i.e. less robust) spindle diameter. The hub and spindle metrics are all recorded in a Google Docs spreadsheet. See also this thread: Solving a Hub and Spindle Mystery.

Once you’ve arrived at the critical decisions, there are five major optional decisions for you to consider:

What style of front hubs do you want to use

Do you want aluminum or steel A-arms?

Do you want aluminum or steel trailing arms?

What offset ranges will work for your application?

What mounting arrangement do you want for your front sway bars?

There are some dependencies and considerations between these choices and the super-critical decisions listed above, which I will point out as we go.

Optional Decision 1: Front Hub Style
In 1987, the style of hub changed in two important ways. First, the spindle diameter was increased from 31.7mm to 35mm; second, the rotor mounting was changed such that the front rotors were mounted in FRONT of the hub as opposed to the earlier style with the rotor mounted BEHIND the hub. The larger spindle diameter was presumably for increased performance considerations of the more powerful 944S, 944S2 and 951’s that were produced from 1987 onward. The change in hub and rotor configuration is also worth evaluating because the later design makes it much easier to service the rotors since the job no longer required removing the hub in order to remove the rotor. The dependency here is that the later style hubs require the later style spindle, which in turn means no mechanical speedometer cable (these can still be drilled as described above), and you MUST also source a pair of 1987-onward strut housings due to the aforementioned difference in the mounting flange and bolt pattern.

A couple of interesting variations to note are the 1986 951 and the ABS equipped cars. The 1986 951 has a hub and spindle that is unique from any other 944 or 951 spindle. It was designed to accommodate the late strut housing bolt pattern, but it also has the smaller spindle diameter. However, the brake caliper mounting bolts are further apart to accommodate the Brembo 4-piston fixed calipers. Furthermore, the rear face of the hub that adjoins the rotor hat is shallower on the 1986 951 hub than its NA and pre-1986 counterparts in order to accommodate a larger, deeper rotor specifically designed for the Brembo calipers. VDub Engineering claims that they can make a custom hub for the 1986 951 spindle that has the correct diameter and geometry for the spindle and calipers, but is modeled after the later style hubs such that the later style rotors can be used. And, of course, there is no accommodation for a mechanical speedometer cable. As for ABS equipped cars, which became available in 1988, the hubs have a toothed ring which is interesting in that it could be suitably modified to accommodate a DIY wheel speed sensor.

Optional Decision 2: Front A-Arms
First, it’s important to note that it is not absolutely required to replace your A-arms. The early 924 A-arms have the same geometry as all subsequent A-arms up through model year 1988. However, there are some important considerations where acquiring later A-arms from a donor might be beneficial.

There are three primary varieties of OEM front A-arms to consider: early offset steel, early offset aluminum, and late offset aluminum. The steel A-arms came with at least three different mounting arrangements for the front sway bars (none, “early”, and “late”). See Optional Decision 5 below for more detailed discussion of this dependency. The early offset steel and early offset aluminum A-arms have nearly identical geometry, and in this regard are essentially interchangeable. The only major difference is that the front bushing on the steel A-arms are slightly wider than on the aluminum. This is really no big deal because if you plan to run aluminum A-arms, you simply need install a couple of extra washers on either side of the front bushings. A more important consideration is that the aluminum arms are reputed to be weaker and more brittle than the steel arms. For serious performance driving, the steel arms can be box welded to make them even stronger. The flip side is that the aluminum arms are lighter. The late offset aluminum A-arms increase the track of the front wheels, so see Optional Decision 4 below for a more detailed discussion of this dependency.

One other important note about the aluminum arms is that some of the late offset variety do not have rebuildable ball joints. To complicate matters, these arms are stamped with a casting number, but do not have the part numbers that correspond to PET. In order to determine if the ball joint is rebuildable, you have to inspect the bottom side of the ball joint. This requires removing the moisture barrier to determine if there is a removable circlip or not. In my personal experience, the rebuildable arms all have a dark amber colored urethane seal that is best removed by heating and then scraping away, while the non-rebuildable ones have a rubbery grey material that can be scraped or pulled away with a razor knife. The aluminum A-arms are known to suffer from binding issues with the ball pin and the top of the cup, particularly on cars with lowered ride height, so it is probably imperative to select rebuildable arms and replace the ball joints.

It’s also worth mentioning that there are several aftermarket A-arm replacements that are touted as being lighter weight and stronger than any of the stock components. If your suspension plans call for dramatically lowering the car, it is important to be aware that stock ball joints will have incorrect geometry with no provision for correcting, which adversely affects bump steer, and can also cause the ball joint to bind on the A-arm. The aftermarket replacements generally replace the stock ball joints with geometry correcting pins and heim joint arrangements that are stronger and adjustable for ride height. They are generally available in both offsets and routinely sell new for $900-$1600 depending on the supplier. While pricey, if your plans call for significant lowering or rugged driving environments, you may be better advised to budget for a pair of these rather than sourcing, rebuilding, and re-engineering stock units.

Optional Decision 3: Rear Trailing Arms
As with the front A-arms, there are three varieties of OEM rear trailing arms to consider: early offset steel, early offset aluminum, and late offset aluminum. The aluminum trailing arms are MUCH lighter than the steel ones. However, they also require sourcing the rear axle half shafts which can come from any 924S or any 944 1985.5-onward. While they use the same half-shafts as the early trailing arms, the 1989-onward aluminum trailing arms are longer than the pre-89 versions, and consequently have the late offset. So as with the A-arms, you will need to consider which configuration is most appropriate for your project depending on your plans for body work, wheels and total track width. It’s also worth noting that the rear hubs should be sourced as an assembly with the trailing arm.

Optional Decision 4: Wheel Offset
As noted above, your choice on A-arms and trailing arms will have an impact on wheel offset. The two major things to consider are whether or not you plan to use existing wheels, and whether or not your body work can accommodate later offset. For example, if you need to keep your existing wheels and you plan to retain original 924 wheel arches on the body, it is unlikely that the late offset suspension components will work for you. Conversely, if you’re doing a wide body conversion of any type (stock 944, D-Prod, CGT, GTR, etc.), the late offset suspension could be a tremendous benefit by widening the track and minimizing the size of wheel spacers required to fill up the arches.

Optional Decision 5: Front Sway Bar Mounting
As noted above, there are at least three different mounting arrangements I’ve identified on 924/944 variants. If your original car did not come with sway bars, your original A-arms can be modified with a little bit of effort to accept any of the mounting arrangements, but you might be just as well off getting A-arms from a donor to match the sway bars you wish to run.

The biggest consideration regarding sway bars is that the early style sway bars have swing arms that are nearly twice as long as the later style bars. This is critical. According to Herb Adams (author of Chassis Engineering), reducing the swing arm length in half results in an effective torsional resistance that is doubled, assuming the same diameter and construction of bar. Consequently, a late 23mm sway bar probably has nearly twice the torsional resistance of the earlier 23mm sway bar due to the shorter swing arms. For this reason alone, in my opinion, the later sway bars are much more desirable than the early variety. Furthermore, many of the later style bars are also tubular, which reduces their weight. So you will need to consider what approach you may wish to take initially or in subsequent stages regarding your sway bars to determine which A-arm might be best for your intended application.

It is also worth noting that there are many available aftermarket sway bars that are fully adjustable, and completely compatible with the 944 style mounting arrangements. They usually cost $500 or more, but with the benefit of a front and rear setup that is totally tunable by adjusting the drop links in order to induce the desired amount of over- or understeer for a given track or driver. So before you plunk down big dollars for that 968 tubular setup, you may wish to evaluate the aftermarket alternatives, which offer greater flexibility and tuning.

Other Considerations and NotesBrake Calipers
As noted above, there are two primary styles of brake calipers: single-piston floating and four-piston fixed, the latter obviously being the superior arrangement. Of course, there are also aftermarket upgrades available from Rennbay (Wilwoods). If you source non-951 or non-M030 spindles, you can run any of the single-piston floating calipers, and you can later upgrade to the OEM four-piston Brembos or the aftermarket Wilwoods using an adapter bracket. As for rear brakes, the bolt pattern is identical for all styles of calipers on all styles of trailing arms, so there really aren’t any constraints I’m aware of regarding your selection at the rear.

Brake Booster & Master Cylinder
According to some posts I found by gohim, the piston size on all 5-lug master cylinders is the same. I have confirmed this to some degree by looking at a variety of 924, 931, 944, and 951 master cylinders. There are minor differences in the hardline port arrangements, and the earlier cars used a brake light switch in the reservoir cap, while the later cars used a switch installed into a port on the master cylinder. You may wish to consider switching to a mechanical brake switch on the pedal mount (it’s already pre-drilled for one) to eliminate the sometimes troublesome pressure based switches on the reservoir or cylinder.

The boosters themselves might have different servos, perhaps indicated by the different part numbers in PET, but I cannot verify this from personal experience. It appears to me that these are all pretty much interchangeable between the various setups, but it may have an impact on peddle feel and power assist. Lacking good empirical testing and confirmation, it may be best to assume that the brake master and booster should be matched to the calipers you intend to run. Personally, I don’t believe this to be an absolute requirement, but you should certainly consider and be aware of the possible ramifications.

The other major consideration when performing a conversion is that the 4-lugs were setup with a diagonal split, whereas the 5-lug systems were setup with a front-rear split. It should be a simple matter of changing the stock tee on a 4-lug system to switch over to a front-rear split.

Handbrake Setup
An important consideration not to overlook is the various handbrake configurations. The 4-lug handbrake setup uses different attachment points at the rear drum than do the 5-lug setups within the rear disc hat. Consequently, you MUST source an unmolested two-piece handbrake cable from a donor car. To further complicate things, there are three different lengths of trailing arms: the steel ones (same length as 4-lugs), the early offset aluminum variety, and the late offset aluminum variety. Interestingly, I have discovered that the two-piece cable is the same length for all varieties of 5-lug suspensions. The difference is in the linkage that attaches the cable itself in the cabin to the handbrake lever. There are three different sizes, a short one for steel a-arms, a medium one for early offset aluminum a-arms, and a long one for late offset aluminum a-arms. The point here is that it doesn’t matter which five lug donor you get your two-piece cable from, but you MUST get the in-cabin linkage for the lever that is appropriate to the a-arms you intend to use.

5-lug conversion parts can be sourced from donor cars for as little as $200, but as much as $500 or more, depending on the seller and exactly what you need to complete the conversion. Expect to pay more if you intend to go the mix-n-match route, vs. whole-car-donor route. A simple whole-car swap is definitely doable over a weekend, provided you have all of your donor parts pre-assembled, have sourced all of your wear items (rotors, pads, bushings), and have already rebuilt the calipers

Suppliers
I have sourced many five lug conversion parts from the following eBayers/Rennlisters/Pelicaners: porsche.racer, plyhammer, and

Adjustable Bump SteerChoices
Aftermarket, DIY

Evaluation Criteria
The 924/944 platform suffers from inherent bump steer, as discussed in the 924.org technical section. When lowering the ride height, bump steer will be further exaggerated. The technical section describes a DIY approach to fixing the problem, but there are now aftermarket options available as well. Both Rennbay and Paragon carry a couple of ride height adjusting ball joint kits (one with a stock 17mm diameter pin, the other with a beefier 19mm pin, requiring machining of the pinch socket on the spindle). In addition, Elephant Racing sells a tie rod bump steer kit that includes a rod-end ball socket, a reinforcing yoke (so that the main tie rod bolt doesn't carry the full load), and a set of three different-sized shims that can be placed in a variety of combinations above or below the spindle arm to help level the tie rod arms. The yoke requires very minor tweaking of the end of the spindle arm. On mine, I had to remove about 1mm from each side of the rounded spindle arm, and I also had to remove probably less than 1mm from the flat horizontal surface in order to get the spindle to fit inside the yoke and squeeze in all three shims. Easily done in about 10 minutes at the bench grinder.

Evaluation Criteria
If your car doesn’t have ‘em, get some and add them. Simply put, they will help reduce body roll during hard cornering. Given the wide array of OEM parts available, sway bars are probably one of the least expensive and easiest methods of improving your suspension. It’s important to recognize that the anti-roll characteristics should be matched between front and rears, so as not to unintentionally introduce under- or over-steer characteristics. An instructive reference is Jim Pasha’s article on Suspension Preparation, and in particular the table where he lists out the factory-available option. It is probably safe to assume that a factory combination is a good place to start when adding or upgrading sway bars.

In a nutshell, there were two main varieties available: the earlier style that were suspended from the front a-arms, and the later style that mount directly to the a-arms. Within the late style, there are both solid and tubular varieties available, presumably each with its own anti-torsion rate. Consequently, one cannot assume that a 23mm tubular bar is necessarily better than a solid 21mm bar. They could actually have similar effective torsional resistance rates! So far, I haven’t found any empirical data on the torsional resistance of the factory bars. More importantly, the later style had much shorter swing arms, nearly half the length of the early style, which effectively doubles their anti-torsion rate…so a late 23mm bar is probably better than an early 23mm because of the difference in the swing arm configuration. In my opinion, if you’re looking to upgrade your sway bars, you should definitely consider sourcing the later style for this reason alone.

The available factory sway bars are pretty much interchangeable as far as I can tell between the 924 through the 968, so identify your desired stiffness and source your front and rear bars accordingly from a donor car. Alternatively, there are several adjustable aftermarket bars to consider, particularly targeted at the 944 crowd, so don’t overlook those as options. Be sure to consider how the sway bars will interact with the front spring and torsion bar setup you plan to run. Just be aware that if your car does not have stock bars, some minor fabrication will be involved in order to mount a set. Also, if you want the later style sway bars, you may need to change or modify your early style a-arms, or replace them with late style a-arms with the mounts already built in.

Evaluation Criteria
OEM rubber bushings provide a nice ride, but they also deflect quite a bit under performance-oriented driving. Deflection in the suspension basically means that all of the work you did to set it up correctly goes out the window because the geometry changes…sometimes radically…under extreme driving conditions. Replacing as much rubber as possible with firmer components – whether polyurethane, delrin, monoball, or solid mounts – eliminates deflection and thereby improves handling, but at the cost of ride comfort, noise, and vibration. As a result, you must realistically evaluate what your intended driving conditions will be to determine whether delrin/poly bushings are sufficient, or if you want to go all out with a monoball/solid race setup.

As with 5-lug options, at first glance, there seems to be a dizzying array of options for bushings. I’ve attempted to capture some of this information in a spreadsheet tool I put together recently. In short, once you’ve upgraded to a 5-lug system, you can upgrade the major suspension bushings with race-oriented monoball and solid replacements, with performance oriented Delrin or Polyurethane, or with OEM rubber replacements. Currently, my research indicates that there is no single vendor that has a complete package available, and this problem is exacerbated by the plethora of sway bar options available across the model range.

For evaluation purpose, then, I’ll break this into two primary categories: primary suspension bushings, consisting of A-arms, trailing arms, and the torsion carrier; and secondary suspension bushings, consisting of all of the sway bar attachment points.

Primary Suspension Bushings
For primary bushings, there are two major manufacturers of race-oriented monoball and solid bushings: Racer’s Edge and Elephant Racing. From what I can tell, product offered by Lindsey Racing and Tarett are identical both in terms of price and photos to those offered by Elephant. Paragon Products carries Racer’s Edge. These two major lines provide monoball replacements for the A-arm front and A-arm castor block, as well as the rear trailing arm bushing. They also offer solid bushings for the rear torsion carrier, the spring plate, and the upper torsion mount. Racer’s Edge is the only supplier I’ve found that also make a solid upper torsion wing mount.

I was surprised to learn that there appears to be very few vendors offering Delrin or Poly bushings for ALL of the primary bushing points. You can get polyurethane A-arm bushings (front and rear) from Powerflex, but the other vendors only offer the A-arm rear bushing in Poly or Delrin. Likewise, the rear trailing arm bushing is only available in Poly from one vendor (Paragon). A couple of them carry poly bushes for the spring plate. But there is very spotty coverage, not a comprehensive set available.

Secondary Suspension Bushings
The sway bar situation is even worse. Racer’s Edge make sway bar bushings in three sizes for the fronts, and three sizes for the rear, but these are ONLY for the main bushings on the center of the sway bar itself, and do not include all of the drop links and attachment points. The bottom line is that for now, until some vendor steps up to the plate, you will have to use the above-linked spreadsheet to sort out which bushings you want to source based on exactly which sway bars you have, and you may also end up with stock OEM replacements for some of these secondary bushings regardless of your exact configuration. It’s also worth pointing out that Racer’ Edge makes a solid rear sway bar retaining bow with Delrin inserts, and there are a handful of improved heim joint style drop links available for the rear sway bar attachments.

If upgraded bushings are not available for your particular configuration, it is still beneficial to change the old worn out stock rubber units. New, they are actually quite decent for spirited street driving, while in my experience the polyurethane tend to be just a tad harsh and noisy (maybe because I have to drive on roads cratered from being covered in snow and salt six months out of the year!). In any event, polyurethane bushings are widely appreciated in the AutoX and DE circles. Expect an investment any where from on-par with stock rubber to maybe double.

Evaluation Criteria
It is important to note that some of the late 944/951 strut housings are NOT rebuildable, so you have to be careful in selecting the donor car for strut housings if you wish to replace the cartridge with a new or uprated version. It's also worth noting that several aftermarket resellers provide a complete front strut assembly with their coil-over kit, and most of these that I've seen are designed for the later strut housing bolt pattern.

Strut inserts are a matter of taste, driving habit, driving conditions and venue. I don’t have enough experience to comment on viable options, other than to say that the KYB stock replacements are decent enough for moderately spirited street driving. The other option here is for aftermarket front springs with adjustable ride height, for lowering the ride and increasing stiffness. Until we have more comprehensive research on this topic, the best reference for upgrades is 924RACR’s Race shock/strut options - Bilstein front, QA1 rear

Evaluation Criteria
Same as above. Some have expressed interest in rear coil-overs in favor of torsion bars, but I’m not conversant enough to comment on the pros and cons of the two approaches. Some responses have indicated concern over the ability of the rear shock supports to withstand the abuse imparted by rear coil overs. We will need input from the experts here. Until we have more comprehensive research on this topic, the best reference for upgrades is 924RACR’s Race shock/strut options - Bilstein front, QA1 rear

TODO: I would like to develop some basic information about matching spring rates between the various torsion bar sizes available and front springs, as well as rear coil-overs, and torsion+rear coil-overs.

Evaluation CriteriaJim Pasha’s article on Suspension Preparation indicates that OEM torsion bars are available in the following sizes: 22mm, 23.5mm, 24mm, 25.5mm, 25.5mm tubular, and 26.8mm tubular. Refer to his article to determine which torsion bars your car has, as well as potential OEM alternatives to source from your donor car. In addition to OEM options, there are a variety of aftermarket upgrades ranging in sizes all the way up to 34mm, Weltmeister being the notable supplier.

If you’re planning to dramatically lower the ride height, you will probably want to re-index your torsion bars. If you are going to source replacements from a donor car, it is absolutely essential that the person removing the bars mark them as to which side of the torsion carrier they came from (left or right). They are not marked from the factory, and once out of the torsion carrier it is exceedingly difficult to determine which is which.

Changing or re-indexing torsion bars can be a messy, complicated job that involves lowering and disassembling most of the rear suspension. If you’re doing a 5-lug swap, now is the time to address your needs & desires with respect to torsion bars; it may be easier to simply source a complete torsion carrier assembly from a donor car, but this may not obviate the need for re-indexing depending on your ride height objectives. Also, keep in mind that the torsion bars need to be balanced with your front springs. If they are out of balance, you will have trouble with under- or oversteer. I don’t know the formula for matching spring rates to torsion bars, but in my UWB project, I will be installing 30mm torsion bars with 400# front springs. Need more input from the experts here.

This thread provides a comparison of the spring rates between torsion bars and rear coil-overs, but it is still missing the matching front spring rate requirements.

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Camber plates may not be legal in all racing classes, so be sure you understand the regulations if you plan to race. The stock upper strut mounts with their large rubber bushing produce significant deflection under hard cornering, particularly on old warn bushings. At a minimum, the upper strut bushing and all of the associated bearings and seals should be replaced with new units.

There are at least two varieties of aftermarket camber plates available: solid monoball, and adjustable. The solid monoball style replaces the soft rubber bushing with its needle bearing insert. The adjustable style allow for quick, trackday adjustments of the front camber, which can be very helpful for maximizing tire tread life on cars that see both street and track duty. For pure racers, if the regulations allow, adjustable camber plates can also allow for easier trackday tuning.

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The consensus view is that a strut brace on the 924/931 is largely unnecessary for all but the most demanding track applications. Furthermore, there aren’t too many commercial options that will work on a stock 924 due to interference with the charge tube (on a 931) and intake manifold. The Weltmeister or Racing Dynamics bars are a bit pricey (in the $300+ range); putting one on a street car with the stock upper strut bushings is pretty much useless, so unless you only care about the cosmetic appearance, your money would be better invested elsewhere.

There are at least two options for lower chassis braces targeted toward the 944 crowd, one from RSBarn and one from Brey-Krause, but I’ve not found any independent testimonials discussing their merits.

In my opinion (and it is JUST opinion), based on what I’ve read about torsional deflection in chassis, I suspect that a simple singular bar across the chassis or strut towers do very little to add to rigidity. On the other hand, I suspect that the X- or Z-style cross braces that were used in the GTR cars are probably MUCH better at resisting torsional deflection because of the triangulation they provide across the chassis. Currently, I am unaware of any aftermarket replicas of the GTR style braces.

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If you’ve converted over to 5-lug suspension, this opens up some options for improved braking. The pre-87 944 single-piston calipers with their larger pistons are an improvement over the standard 931 units, and should be direct bolt-on. The 951 calipers are highly desirable due to the 4-pot Brembo cores; the rears are direct bolt on, but the fronts require either the 951 spindles or custom fabricated adapters. Be aware that switching to the 951 spindles affects other things like the geometry, strut assemblies, speedo cable, a-arms, etc.

Steve Bassington is currently working on converting to the 968 setup in order to take advantage of the ABS features. There are also at least two commercial options I am aware of: the big brake upgrade from Lindsey Racing, and the Wilwood conversion kit from Rennbay. These fall into the $800-$1200 range depending on what you select. I have not yet collected all of the variants into a single place, so this will need some work to validate.

Another vendor worth noting is VDub Engineering. They specialize in Porsche 944/951 brake conversions for Volkswagens. They claim that they can manufacture adapters that will allow essentially any late model Porsche calipers to be bolted onto the 944 spindles. These are not off-the-shelf components, and would are essentially manufactured as one-off projects, but I have found VDub’s Lanner Khan to be extremely knowledgeable and helpful, so I have no reason to doubt their ability to deliver.

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Need more research and input from experts here. I do know that there are different thicknesses and diameters of rotors that have to be considered, but have not exhaustively documented the details. I also know that there is a lot of debate regarding the pros and cons of solid, vented, cross-drilled, and slotted rotors. While I haven’t yet reached my own conclusions, the gist of what I have gathered is that those really fancy slotted or cross-drilled rotors might look great, but may not offer the best braking performance. Be sure you do your own research and understand the braking characteristics you want, and weigh that against the bling factor you’d like to achieve. For me, I’d much rather have a “plain jane” rotor that stops better and exhibits little fade, than to have a great looking rotor that craps out after the first lap!

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For aggressive street driving, I have had excellent success sourcing tires from Tire Rack. So far I have opted for the Ultra High Performance All Season tires, and my selections have been Yokohamas for my NA and Kumhos for the 928 and 924S. As for race tires, I have no expertise in this area, and I suspect it depends a great deal on your venue. There are other threads on the topic, so we’ll need to distill that information here at a later date. Any volunteers?

References
Innumerable tire threads, search based on your interest

Suppliers
TireRack, many others

WheelsChoices
Stock, aftermarket, custom forged

Evaluation Criteria
From a performance perspective, stock wheels from Porsche are actually not that bad in terms of weight. A general principle is that smaller diameter wheels provide quicker acceleration, so don’t be eager to go for the 18’s with low profile tires unless cosmetics are more important to you than acceleration. Conversely, 15’s are about as small as you can go on the five-lug setup, and 16’s may be required if you plan to run larger calipers, such as the Brembos or Wilwoods. There are a few sources of light weight forged aluminum wheels designed for racing, and these can be had for as low as $1800 per set, on up into the $5000+ range.

BODY
A lot of newbie posts ask the question “how can I get more HP”. A better question would be, “How can I improve my power-to-weight ratio?” Output is only one side of that equation. Weight reduction is often overlooked by the neophyte. What’s more, a lot of weight reduction can be achieved with nothing more than time and effort, so it’s essentially free performance boost. The rule of thumb is that every 100 pounds of weight is the equivalent of a gain of 10 BHP. In light of these factors, one could even argue for putting this section at the top of the list, in terms of the biggest bang for the buck!

The stock 924/931 weighs in at approximately 2500 pounds, which is pretty light compared to similarly sized modern cars (e.g., the Pontiac Solstice weighs in around 3200 pounds). To help you establish a benchmark for your project, the consensus view is that a street car can lose around 200 pounds, and some track cars have been radically lightened by nearly 400 pounds (net). There are lots of threads on how, what, and where to lose weight, so I’ll just point out the highlights here. Also, I haven’t tallied up the weight of all of the various components, nor am I professing the following to be an exhaustive list, but it should give you some ideas

Chassis LighteningChoices
Stuff that is free, swaps that will cost money

Evaluation Criteria
Obviously, these will add weight, but if you’re going to be converting your car to all fiberglass, you may want to add in a margin of driver and passenger safety. For track cars, your choice will be mandated by the sanctioning body. Obviously, a simple roll bar will be less expensive and lighter weight than a full roll cage.

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You may want to consider moving the battery to the rear of the car for better weight distribution. At least one member selected the left rear wheel arch location due to the nature of the venue he intended to frequent. Be aware that for race applications, there may be specific safety requirements regarding the gauge of wire you select, as well as automatic and externally mounted cutoff switches. While you’re at it, you might consider switching to a lightweight dry cell battery for add’l weight savings.